1. Field of the Invention
This invention relates to no-carbon copying paper, more particularly to improvements in no-carbon copying paper produced by combining microcapsules containing a colorless electron-donative organic compound with activated clay which is an adsorbent material capable of adsorbing said organic compound to develop color.
2. Description of the Prior Art
No-carbon copying paper is well known, and, for example, those disclosed in U.S. Pat. Nos. 2,712,507, 2,800,457 and 2,730,457 are produced by combining microcapsules containing a colorless electron-donative organic compound (hereinafter referred to as "color former") with an electron-attractive adsorbent material (hereinafter referred to as "color developer") which adsorbs said color former to develop color.
No-carbon copying paper is composed of a sheet (called "top sheet") produced by providing a support with a layer of microcapsules containing a color former, a sheet ("under sheet") produced by providing a support with a color developer layer and, if necessary, one or more sheets ("intermediate sheets") inserted between top sheet and under sheet, which intermediate sheets are produced by providing the right side of a support with a color developer layer and its back side with the above-mentioned microcapsule layer. The term "color developer sheet" hereinafter used refers to both above-mentioned under sheet and intermediate sheet.
Microencapsulation has heretofore been carried out by a coacervation method, an interfacial polymerization method, an in situ method, or the like, and as the color former, there are used malachite green lactone, crystal violet lactone, benzoyl leucomethylene blue, rhodamine B lactam, 3-dialkylamino-7-dialkylamylfluoran, 3-metyl-2,2-spirabi (benzo-[f]-chromene), and the like. As the color developer, there are generally used, for example, solid acids such as acid clay, activated clay, attapulgite, zeolite, bentonite, and the like; phenol resins such as p-tert-butylphenol resin, p-phenylphenol resin, p-octylphenol resin, and the like; organic compounds such as succinic acid, tannic acid, malonic acid, maleic acid, gallic acid, and the like; and aromatic carboxylic acids such as benzoic acid, salicylic acid, substituted salicylic acids, naphthoic acid, diphenic acid, and the like and metallic compounds thereof.
Among these color developers, those practically used from a consideration of their characteristics are activated clay, phenol resins and substituted salicylic acids (or their salts).
Among these color developers, organic color developers such as phenol resins, substituted salicylic acids, and the like are disadvantageous in that they tend to be decomposed by sunlight to change color to yellow and in that the colored characters have so poor solvent resistance that they are defaced. On the other hand, the solid acids as inorganic color developers are free from these disadvantages and can improve the shelf life of the coated paper. Activated clay used as the color developer is prepared by treating acid clay or analogous clay with a mineral acid to dissolve the acid-soluble basic components such as alumina, iron, and the like, and thereby adjusting its surface area to 200 m.sup.2 /g or more, as described in Japanese Patent Publications Nos. 2,373/1966, 7,622/1966, 8,811/1967. etc. Activated clay is amorphous from X-ray observation, has a large surface area, and is very different in properties from pigments for coating ordinary paper.
Kaolin, a representative clay for coating paper, shows fluidity when dispersed in water up to a concentration of 70% or higher. On the other hand, activated clay becomes highly viscous and loses fluidity to gel at a concentration of about 45%. At present it is desired from a viewpoint of productivity and economy in energy to apply a clay for coating paper in the form of a highly concentrated coating color. However a highly concentrated coating color of activated clay is very difficult to prepare for the reasons mentioned above, and therefore, in the existing circumstances, there is mainly employed an air knife coater method using a coating color of a low concentration.
When kaolin and activated clay were individually added in an equal amount to starch and latex and each of the resulting mixtures was applied to paper, after which the surface strength and the stains on the blanket at the time of printing were measured, there were obtained such results that in the case of using kaolin, much higher surface strength and less stain on the blanket were brought about than in the case of using activated clay. When the adhesive is further added to activated clay, the surface strength is improved and the stains on the blanket is reduced a little. However the depth of developed color is decreased so that the aptitude as a color developer sheet for no-carbon copying paper is lost.